Eyes of Molluscs and Arthropods. 699 



nothing in the principal upon whicli the compound eye is constructed 

 to prevent its posscssor from seeing with clearness and precision, by 

 means of inverted images, either a landscape or small neighboring ob- 

 jects. There seems to be no reason to deny that many Inseets have 

 developed at least one of these faculties to a special degree. On the 

 other band there seem to be many Arthopods, with well developed com- 

 pound eyes, which are able to see, but not to perceive objects, or at least 

 only a very few objects, — those with which they are brought into the 

 closest relation. Then again there are probably many Inseets which 

 can only distinguish light from darkness. The diiference in vision is 

 due more to the powers of association than to Variation in the structure 

 of the eye. 



Chapter V. General Remar ks upon other Oroups. 



The knowledge we possess eoncerning the visual organs of Coe- 

 lenterates is sufficient to warrant a comparison between the structure 

 of their essential Clements and those of Mollusca and Arthropods. As 

 far as I know, there are no structures in the Coelenterata that might 

 be regarded as isolated ommatidia, but that such organs do exist, or 

 have existed once, is quite probable, since their eyes are formed on the 

 same plan as those of the Mollusca, i. e. of collections of ommatidia 

 which, in the less perfect condition, must have been irregularly scat- 

 tered about. The sensitive layer, which appears to form a retineum, 

 consista of colorless retinophorae, whether single or double is not 

 knowu, surrounded by pigment cells, or retinulae. This structure is 

 found in the sensitive layer of all the various forms of Coeleuterate 

 eyes. The only difference of any moment is that the retinophorae are 

 much smaller, and in shape and general appearance resemble more 

 closely ordinary sense cells than is the case in Mollusca, where the 

 bright, refractive Contents give them the appearance of gland cells. But 

 in the Coelenterates anything similar to the rods of MoUuscan eyes 

 has not as yet been described. But I think we may safely conclude 

 that what has been, heretofore. regarded as a vitreous body is, in re- 

 ality, a cuticular thickening containing a highly specialized part of the 

 reti a terminalia. Such appears to be the case with the minute in- 

 vaginate ocelli, described by Carrière, situated near the large lenti- 

 culate eyes of Charyhdaea marsupialis^ which can be compared with the 

 invaginate eyes ^i Arca (PI. 30, fig. 43). The vitreous layer in the 

 large, lenticulate eyes of the same species is, in ali probability, a layer 

 of retinal rods. 



